This term came about through a very popular cartoon of a helmet topping off a pair of boots to signify the veteran with just a short time left in his enlistment and keeping a REALLY low profile, turtle-like, trying to make it out alive. Among VietNam and older veterans a challenge term by which infantrymen recognized each other.

When a veteran infantryman would meet another veteran, the question would be asked: "Are you a turtle?". To which the challenged veteran would have to either reply "You bet your sweet ass I am!" or have to buy the challenging veteran a drink.

a metaphor for a dude with an exoskeleton that provides said dude with the illusion of invulnerability. Said Dude is also sometimes said to be endowed with a long turtle-neck, which causes him to move a bit slower and with relatively more confidence than other dudes.

Can also be used as an adverb, which is a recently new development and is expected to catch on quickly